The present invention relates to an electroviscous fluid and, more particularly, to an electroviscous fluid which has an excellent insulating property, which is usable in a high electric field and which generates an excellent electroviscous effect. The electroviscous fluid is useful for a vibration isolator and a power transmission which are electrically controllable.
The electroviscous fluid is a fluid whose apparent viscosity rapidly and reversibly changes depending on ON and OFF (change) of an electric field applied.
The electroviscous fluid is generally produced by dispersing polarizable particles in an electrical insulating liquid. It is considered that an electroviscous effect is produced as follows. When an electric field is applied to the electroviscous fluid, the particles dispersed in the liquid are polarized and agglomerate in the form of a chain by electrostatic attraction based on the polarization. As a result, an electroviscous effect is displayed.
As such electroviscous fluid, many kind of electroviscous fluid containing particles which have adsorbed a polarizable solvent such as water, as the dispersed particles, are conventionally known. As a fluid containing particles having no adsorbent, as the dispersed particles, have been proposed a fluid containing semiconducting particles such as polyacene quinone (Japanese Patent Application Laid-Open (KOKAI) No. 216202/1986), a fluid containing electrical conductive particles such as particles of aluminum covered with an electrical insulating film (T. Sasada et al: Proc. 17th Japan Cong. Mater. Res., 228 (1974)), a fluid containing composite particles having a three-layered structure with an electrical conductive layer such as a metal layer disposed as an intermediate layer (Japanese Patent Application Laid-Open (KOKAI) No. 97694/1988), a fluid containing dielectric particles such as barium titanate (Japanese Patent Application Laid-Open (KOKAI) No. 17585/1978), etc.
Furthermore, it has been also proposed a fluid containing particles having different particle sizes as the dispersed particles in order to enhance the redispersibility of the deposited particles (Japanese Patent Application Laid-Open (KOKAI) No. 160094/1991).
However, a fluid containing particles which have adsorbed a polarizable solvent such as water is disadvantageous in that when it is exposed to a high temperature for a long time, the adsorbent evaporates and the electroviscous effect is lowered.
As the fluid containing particles having no adsorbent, in the case of using the fluid containing composite particles, since it is difficult to cover the particles with a uniform thin film, a sufficient electroviscous effect is not always obtained. In the case of using the fluid containing only dielectric particles, since the specific gravity of the particles is generally high, the particles may disadvantageously sediment. In the case of using the fluid containing only semiconducting particles, the insulating property is insufficient, so that application of an electric field sometimes causes dielectric breakdown, and as a result, it is difficult to obtain a sufficient electroviscous effect. In the case of using the fluid containing particles having different particle sizes, the insulating property of the particles as a whole is sometimes lowered depending on the kinds of the particles, so that dielectric breakdown is caused in a low electric field, and as a result, it is difficult to obtain a sufficient electroviscous effect.
As a result of the inventors' studies to solve the above described problems, it has been found that by dispersing semiconducting particles having a comparatively large average particle size and dielectric particles having a smaller average particle size than that of the semiconducting particles in an electrical insulating liquid, obtained electroviscous fluid is not deteriorated at a high temperature, has an excellent insulating property, is usable in a high electric field and shows an excellent electroviscous effect. The present invention has been achieved on the basis of this finding.